There are actually several in Abra-Castrophe, but possibly the most crowning one is when Timmy reveals the existence of his godparents, and then Crocker realizes that will set them free and leave him powerless. The look on Crocker's face was, well, a very crowning moment of shock.
In addition, Timmy's actions afterwards shows that he knew how to take complete control of a disaster. His gambit to stop Crocker by losing his fairies in order to save them from his evil teacher and de-power Crocker led to him gaining access finally to the Fairy-versery Muffin and making his rule-free wish: to have Cosmo and Wanda back as his fairy godparents.
Cosmo near the end. He's freaking out about Crocker taking over the world and (more importantly) taking Wanda as a magical power source. He knows he's not the brightest, so it looks like he temporarily abandons Timmy in his time of need. But when Cosmo returns, he's ripped and proceeds to pound Crocker to a pulp for kidnapping his wife. A short time later, this awesome moment is slightly dampened by Cosmo ending up in the same magic scepter that Wanda is, but he still deserves credit for a rather impressive attempt to save the day.
Actually, Timmy's whole fight against Crocker. He doesn't have the ability to wish his way out of the situation. Instead, he needs to rely on the variety of gifts from the beginning of the movie in order to counter Crocker's attacks. And he holds his own fairly well.
At the very end, Jorgen arrives and says that, rule-free wish or not, Timmy still revealed his faries and has to have them taken away. Upon being asked if he has any last words, Timmy replies "Think fast!" and throws the Forget-Me-Knob at him, making him forget that that's why he was there and allowing Cosmo and Wanda to trick him into reassigning them to Timmy.